Photo- and Collision-Induced Isomerization of a Charge-Tagged Norbornadiene–Quadricyclane System

Molecular photoswitches based on the norbornadiene–quadricylane (NBD–QC) couple have been proposed as key elements of molecular solar thermal energy storage schemes. To characterize the intrinsic properties of such systems, reversible isomerization of a charge-tagged NBD–QC carboxylate couple is investigated in a tandem ion mobility mass spectrometer, using light to induce intramolecular [2 + 2] cycloaddition of NBD carboxylate to form the QC carboxylate and driving the back reaction with molecular collisions. The NBD carboxylate photoisomerization action spectrum recorded by monitoring the QC carboxylate photoisomer extends from 290 to 360 nm with a maximum at 315 nm, and in the longer wavelength region resembles the NBD carboxylate absorption spectrum recorded in solution. Key structural and photochemical properties of the NBD–QC carboxylate system, including the gas-phase absorption spectrum and the energy storage capacity, are determined through computational studies using density functional theory.


Computational Methods
The photochromic properties of the NBD-QC carboxylate system were theoretically studied using density functional theory (DFT) and its time-dependent analogue (TD-DFT). All calculations were performed using Gaussian16. 1 Structures resulting from geometry optimizations were confirmed as energy minima or transition states through harmonic frequency analyses (having zero or a single imaginary frequency, respectively). The Gibbs free energy calculations include the zero-point vibrational energy and thermal contributions evaluated at a temperature of 298.15 K and a pressure of 1.00 atm.
The geometries of the NBD and QC carboxylate were optimized in vacuum using the range-separated hybrid functional CAM-B3LYP 2 and the Pople style 6-311+G(d) basis set [3][4][5] with a negative charge and a spin multiplicity of one. This method has previously provided spectroscopic properties of similar systems that are in good agreement with experimental results. [6][7][8][9][10] The adiabatic electron detachment energies were obtained using the same method and basis set.
Conformational samplings of the two structures were performed by employing the Confab algorithm implemented in Open Babel 2.4.1 11 with an energy cutoff of 50.0 kcal/mol and a RMSD cutoff of 0.3 Å. This resulted in 5-6 conformers, which were reduced to 1-2 distinct conformers after geometry optimizations in both vacuum and methanol followed by RMSD calculations using the Kabsch algorithm 12,13 with a tolerance of 0.3 Å. The solvent was treated as a dielectric medium through a static and an optical dielectric constant ( st = 32.61 and op = 1.766, respectively) using the IEF-PCM continuum solvation model. 14,15 Subsequently, TD-DFT calculations for the lowest-energy conformers were performed in order to simulate the UV-Vis spectra shown in Figure S2. For the transition state (TS), calculations were also preformed in both vacuum and methanol and the imaginary frequencies were confirmed to correspond to the breaking/forming of the π-bonds in the NBD isomer and the forming/breaking of the σ-bonds in the QC isomer. Intrinsic reaction coordinate (IRC) calculations confirmed the connection of the TS to the NBD and QC isomers. The energy S2 of the TS allowed us to calculate the energy barrier of the thermal back-reaction going from the QC isomer to the NBD isomer (∆H ‡ QC→NBD and ∆G ‡ QC→NBD ). It should be mentioned, that we have performed calculations with both unrestricted DFT (UDFT) and restricted DFT (RDFT), due to previous studies on similar systems 6,8 reporting spin contamination of about 1 in the area of the TS, as the S 0 and T 1 states become degenerate. However, for this system we observed no spin contamination and obtained similar results for the two methods. Furthermore, the IRC calculations resulted in an energy landscape with a proper saddle point having a finite curvature, which further indicates that the S 0 and T 1 states of this specific system are not degenerate.

Molecular geometries
In this section, we present the geometries and energies of the lowest-energy conformers    Intrinsic Reaction Coordinate Vacuum Figure S1: IRC calculations at the CAM-B3LYP/6-311+G(d) level of theory for conversion of NBD carboxylate (left) to QC carboxylate (right). Energies are relative to the NBD carboxylate isomer in methanol.

Simulating UV-Vis Spectra
In this section, we describe the methods used to simulate the UV-Vis spectra of NBD carboxylate. The spectrum is simulated assuming the bands can be represented by Gaussian functions with intensities proportional to the oscillator strengths. Thus, the UV-Vis spectra can be plotted as the extinction coefficient, , versus the wavelength, λ, using the following where f i is the calculated oscillator strength, λ i is the corresponding wavelength in nm, and ∆ν 1/2 is the full width at half maximum of the Gaussian band (in these simulations ∆ν 1/2 = 0.4 eV = 0.4 · 8065.54 cm −1 = 3226.22 cm −1 ). Furthermore, the constant k is given by k = N A · e 2 2 · m e · c 2 · 0 · ln(10) · ln (2) π = 2.1751 · 10 8 L mol · cm 2 (2) where N A is Avogadro's constant, c is the speed of light, e is the elementary charge, m e is the mass of an electron, and 0 is the vacuum permittivity.